I recently looked at some x86-64 code and was shocked at the number of push and pop instructions. Intel still only have 4 general purpose registers. All 16 registers on ARM are general purpose though you'd be silly to use r13-r16 (stack, link, program counter). That means more stuff in registers less pushing and popping. Just one example of how ARM is a more efficient design.

Reliability is a function of the implementation. So it would depend on the companies designing ARM cores and the RAS features they decide to put in.Intel Itanium e.g has a lot more reliability features compared to the x86 Xeons which again have more reliability features compared to the x86 corei5s used in the desktop parts.

Thanks to all of you for providing many important inputs on this topic. I think, I see majority is voting for ARM and I see very much valid justifications behind the opinion.
Other than performance, the next thing comes to my mind is reliability. What is your opinion about ARM vs. Intel?

To be a serious player, i believe ARM needs to deliver its 64-bit core first. Then we will see who will be winner. But it is no doubt there is better chance for ARM to move up to grab market share from Intel than Intel moves down to grab share from ARM. Because Intel is fighting this war by itself, ARM has entire ARMY around it. This Amry almost includes entire semiconductor companies except Intel and even larger software and tools partner. If Intel wins, the only company benefit is Intel. If ARM wins, there is long list of Companies you could name, starting from Apple, Google, Qualcomm, Samsung, etc. Even those tradition PC/Server companies such as HP & Dell could benefit from it to have alternative choice for their product. I could not imagine how Intel could win this war.
I think Intel should seriously consider build ARM product as well. if ARM is failed, good news. If ARM is successful, Intel could get its share as well.

sorry, I thought you meant core-counts-per-chip - that is, that HPC was pushing to more cores per node. sure, large clusters have lots of cores, since they have lots of nodes. it's not like this is optional: ambitious computing has necessitated for decades.

Well, let's see. The supercomputer used by NASA to discover Earth-like planets has 50,000 cores. I'm assuming it helps if unpredictable tasks can be managed more easily, in this sort of architecture. That core count sounds pretty extreme to me, although I suppose "extreme" is a relative term.
If extreme core counts are not involved then you'd still expect there to be a tradeoff between fewer, higher performing cores, as opposed to more, lower performing, but also lower power consuming cores.
But in general, I'l seeing a lot of arm waving going on here, me included. No one is offering specifics about the difference in the ARM vs x86 architecture. So I'm speculating only based on the popular press reports and common sense.